Mammography machines are used for X-ray examination of the female human breast to detect cancer or other growths. An exemplary version of such a machine is shown in FIG. 1. In general, the machine includes an operator control unit and X-ray generator portion indicated at 20. The portion 20 incorporates the control electronics for the machine as well as the power supply for an X-ray source. THe machine portion indicated at 22 is sometimes referred to as a C-arm assembly and includes a film table 24, an overlaying compression paddle 26 and an X-ray source 28. The C-arm assembly may be rotatable about a horizontal axis 30 for obtaining different angular images. A radiation shield 32 isolates the operator control area adjacent portion 30 from the patient area adjacent film table 24. The C-arm 22 is vertically adjustable, in the position shown in FIG. 1, to accommodate patients of different heights. The table 24 accepts standard X-ray film cassettes for image recording.
In conducting a mammography examination, a patient's breast is placed upon film table 24 and is compressed by compression paddle 26. The compression is required in order to have a substantially uniform density or thickness of the breast typically necessary to provide rather uniform X-ray image density characteristics. In other words, the conventional fixed X-ray exposure techniques generally yield sufficient image quality, i.e., contrast, if the breast has uniform thickness. Such compression procedures are often painful for the patient.
In addition, prior art mammography requires a manual selection of a single kVp and filter for each examination. At best, this may provide proper exposure, contrast and dose for only one portion of the target.
Another disadvantage is that prior art mammography requires manual adjustment of lead collimator blades, which manual adjustment creates two problems. One problem arises if the blades are too close to the skin line, in which case the background image exposure will not be suitable for good viewing. Another problem arises if the blades are left wide open, as is often the case, in which event the intense radiation will produce image fogging scatter.
Still another disadvantage is that prior art mammography involves manual placement of a single X-ray exposure detector. An error in placement of this single detector will produce an incorrect exposure.
Yet another disadvantage is that prior art mammography is subject to film reciprocity which results in increased patient dose if the selected technique factors produce a long exposure time.
A further disadvantage is that prior art mammography involves the use of metal or plastic interspaced grids. Such grids require higher energy X-ray radiation which results in increased patient dose and limited scatter rejection.
In addition to the discomfort associated with prior art mammography examinations attributable to forceful compression, a further disadvantage is a relatively high X-ray dosage necessary to provide sufficient contrast. The X-ray intensity may be set by a sensor located at the film table. The sensor responds to impinging X-rays to adjust the X-ray exposure time so as to set a desired exposure at the sensor location. Such sensor is generally set adjacent a patient's chest wall since that area represents the thickest breast area. Since the level of X-ray intensity for sufficient image contrast adjacent the chest wall is usually much higher than necessary in other areas, the image quality deteriorates toward the nipple due to overexposure of the film.
It is an object of the present invention to provide a mammography machine which overcomes the above as well as other disadvantages of the prior art.